Method of etching a metal which can be passivated

ABSTRACT

A process for etching a first metal which can be passivated, wherein a layer of a second metal which is more oxidizable than the first metal is deposited on the first metal, and the first metal and second metal layer are thereafter immersed in an acid solution.

United States Patent 91 Amouroux et al.

Feb. 13, 1973 METHOD OF ETCHING A METAL WHICH CAN BE PASSIVATEID Inventors: Claude Amouroux, Paris; Gerard Peres, Villepreux; Pierre Valette, Lonjumeau, all of France Assignee: Compagnie Generale Electricite,

Paris, France Filed: Feb. 9, 1971 Appl. No.: 113,924

Foreign Application Priority Data References Cited UNITED STATES PATENTS 10/1971 l/l97l Pn'tchard, Jr. et al. ..156/18 X Fennimore et al ..1 17/212 X Balde ...156/11 X Lepselter ..317/235 Cunningham et al.... 17/212 X Cerniglia et al. ..156/1 1 X Primary ExaminerAlfred L. Leavitt Assistant Examinerl(ermeth P. Glynn AttorneySughrue, Rothwell, Mion, Zinn & Macpeak ABSTRACT A process for etching a first metal which can be passivated, wherein a layer of a second metal which is more oxidizable than the first metal is deposited on the first metal, and the first metal and second metal layer are thereafter immersed in an acid solution.

5 Claims, 10 Drawing Figures PATENTEDFEBIIJISYS FIG.6

FIG. 7

F T 1H2 4 METHOD OF ETCHING A METAL WHICH CAN BE PASSIVATED BACKGROUND OF THE INVENTION The present invention concerns the etching of a metal which can be passivated by an acid bath. This means that the acid bath is capable of etching the metal throughout its thickness, but that this etching is effective only under certain conditions relating to the previl ous treatment of the surface of the metal. This surface mustnot, for example, have been in contact with an oxidizing medium. The metal is said to be passivated when no etching takes place. This passivating can be avoided by various methods, for example, by keeping the metal, from the moment it is produced until it is etched, constantly in a high vacuum or in a strictly controlled atmosphere. These processes are obviously cumbersome and expensive.

The problem is set more particularly, when the acid bath is to act selectively, that is, when it must simultaneously etch the metal in question and not etch another material incorporated in the same part as the metal. When manufacturing transistors by the planar technique, for example, certain areas of a semi-conductor are coated, by a known method, with the object of making non-rectifying contacts with thin layers whose shapes and dimensions must be accurately established. Some of these layers consist of metals which are hard to etch. Now, the contour of these layers is established by the following operations:

1. Depositing a layer extending beyond its final contour;

2. Depositing a photosensitive resin;

3. lnsolation of this resin through a mask so as to make it more resistant on the area which is subsequently to be occupied by the metal, and only on that area. This isolating process is therefore called selective, but this selectivity, which is different from the chemical selectivity previously mentioned, will here be called spatial selectivity;

4. Removing the resin which has not been made resistant', and

5. Etching the metal by an acid bath to which the remaining resin is capable of resisting.

It is quite obvious that the success of such an operational sequence is dependent on the selectivity of the acid bath etching (chemical selectivity). If the metal shows a passivation phenomenon, it is therefore difficult to remedy it by modifying the operating conditions, for example by an increase in temperature or of the duration of the corrosion. The resin would not resist this.

SUMMARY OF THE INVENTION An aim of the present invention is therefore to provide a simple method enabling the passivation of a metal which is to be etched by an acid bath to be avoided.

Another aim is to improve the processes for manu- BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 through 10 are side elevational views of a silicon plate after the successive stages of a process for producing power points on these plates, in accordance with the method of the present invention.

DESCRIPTION OF THE PREFERRED ENIBODIMENT An example of the implementing of the invention will be described herebelow, by way of a non-limiting example, with reference to FIGS. 1-10 which show a small silicon plate after the successive stages of a process for producing power points on these plates. The power points taken from the silicon plate must have a resistive character, that is, they must have no rectifier effect. Moreover, their resistance must be as low as possible. Lastly, they must be very reliable, that is, more particularly, the successive layers must have high mutual adhering power, and must not undergo chemical changes during the course of time. The process which will be described concerns the production of contacts on transistors for ultra-high frequencies on which the dimensions of the emitters are particularly small. The extent of the area on which the emitter contact may be produced is therefore particularly small and the contour of the metal layers ensuring that contact must be defined very accurately. The contacts are produced, of course, after the transistor itself, that is, after the areas having a suitable type of conductivity have been produced within the plate.

At the beginning of the contact production process, the silicon plate 1 is coatedwith a layer 2 of silicon oxide (FIG. 1). A window 20 is opened up in the oxide layer above the area on which the contact is to be made. The method used to accomplish this is wellknown a layer of photosensitive resin is deposited on the oxide layer. This layer is treated or exposed through a suitable mask, so as to make it resistant outside the window to be opened up. The oxide layer is etched by a suitable bath, only the part of the layer corresponding to the window then being removed, because the rest of that layer is protected by the resin. The remaining resin is then removed and the configuration shown in FIG. 1 is obtained. The process which has just been described is called photolithography.

An alloy of silicon and platinum is then produced in this window, this alloy being called here silicide, though its composition is not exactly known by the inventors.

To do this, firstly, the surface of the silicide in the window is deoxidized by quickly passing diluted hydrofluoric acid over it, this being followed by ten minutes of rinsing in deionized water and drying in nitrogen. Then a layer of 3 of 500 angstroms of platinum is deposited by cathode sputtering on all the surface of the plate. Thus, the configuration shown in FIG. 2 is obtained.

The plate is then heated to 500 during approximately 15 minutes so as to form the silicide 4 in the window. Platinum silicide is used to give a low contact resistance. The platinum which has not combined to form silicide is then etched by an acid solution consisting of aqua regia, that is, by a mixture of nitric acid and hydrochloric acid. Etching, which is relatively fast in itself is very long to start up, subsequent to the passivating of the surface of the platinum. It must be carried out as soon as the metal is removed from the enclosure under vacuum in which the cathode sputtering has been effected, and it is desirable to finish it in an ultrasound bath to facilitate the removal of the platinum from the edges of the window. This passivating, which causes a prolonging of the time during which etching takes place, is not a great hindrance, for it is not necessary for this etching to be very selective, the materials provided, other than the platinum, which cannot be etched very much by aqua regia. The configuration then obtained is shown in FIG. 3.

The plate is then covered successively by cathode sputtering of a SOO-angstrom layer 5 of titanium and a lOOO-angstrom layer 13 of platinum (FIG. 4). The function of the titanium layers is to ensure adherence of the subsequent layers on the silicon oxide. Indeed, it will be seen that these layers overlap from the previously opened window. The function of the platinum is to prevent the diffusion of the gold, in a layer which is to be deposited subsequently, towards the silicon, whose electrical properties it would modify. Its function is also to prevent the diffusion of the titanium towards that layer of gold. In order that it may carry out these functions, the thickness of that layer of platinum must be sufficient, but on the other hand, it must subsist only on a well-defined area of the finished transistor. It must therefore be etched by photolithography, that is, in the presence of a protective layer consisting of resin. Its thickness must therefore be sufficiently small. The configuration after the depositing of the titanium and of the platinum is shown in FIG. 4.

The layer of platinum is then limited by photolithography. Firstly, a layer 6 of resin (FIG. 5) is deposited on a well-defined contact area comprising the window previously formed. To do this, the same operations as previously are carried out: non-selective depositing of photosensitive resin, insolation through a mask, rinsing. The configuration then obtained is shown in FIG. 5. Etching the platinum with aqua regia could then be considered, so as to let it remain only in the contact area protected by the resin, which would lead to the configuration shown in FIG. 7. Unfortunately, the passivating of the platinum would cause this etching to take too long too long for the resistance of the layer of resin. The latter would be at least partially etched and the result obtained would be inaccurate. That is why, in compliance with the present invention, first, the layer of platinum and the protective layer consisting of resin are coated with a layer 7 of silver deposited by cathode sputtering. The configuration shown in FIG. 6 is thus obtained. The silver is then etched in approximately ten seconds in diluted nitric acid, and the platinum is etched immediately after, in approximately six minutes by the aqua regia. It can be thought that the surface of the platinum has been depassivated by the silver. When there is no layer of silver, the time during which the etching takes place should have been greater than ten minutes, that is, excessive. The configuration obtained is shown in FIG. 7.

A layer 16 of resin is then deposited in a spatially selective way on the titanium round the contact area coated with platinum. The configuration thus obtained is shown in FIG. 8. Then gold 8 is deposited on the layer of platinum by electrolysis in the following conditions:

The temperature of the bath is 60 C. The plate is prepared, after having been mounted on its support, by a ten minute bath in a 10 solution of sulphuric acid, then by 10 minutes rinsing in deionized water. Electrolysis then takes place for 10 minutes with a current density ranging from 8 to 10 mA/cm In these conditions, a thickness of gold in the order of 2 microns is obtained. The configuration at the end of the electrolysis is shown in FIG. 9.

The layer of resin is then removed. The layer of titanium on the outside of the contact area is then etched by means of a solution consisting half of sulphuric acid and half of hydrogen peroxide. The contact production is then completed by soldering a gold wire on the layer of gold which has thus just been deposited,

this soldering being effected by thermocompression.

In the process which has just been described, the present invention concerns the etching of the platinum after the second depositing of platinum and before the depositing of gold. This process enables contact widths of a few microns to be obtained. However, the present invention is in no way limited to the improvement of this process. More generally, it can be applied to great advantage in each case of a very selective etchingof metal which can be passivated, more particularly when the area in which the metal is etched is to be established with great accuracy. This is done preferably by a photolithography process, the present invention not being connected with the intervention of light. Now, the protective layers used in such a process can have only a limited resistance. It is therefore desirable that the etching should be very selective.

It can evidently be useful to lithograph a metal which can be passivated, with other aims than the production of electric contacts on a semi-conductor.

It is moreover, very evident that the metal which can be passivated and is to be etched, can be a metal other thanplatinum. This metal can be, for example, palladium. Likewise, the auxiliary metal used for depassivating can be other than silver. It can, for example, be copper. The essential point is that the auxiliary metal must be more oxidizable than the metal which can be passivated, thatis, its redox potential must be lower.

More generally, although the elements which have just been described have been considered an advantage in a particular technical situation, other elements ensuring the same functions can be used without going beyond the scope of the present invention.

We claim:

1. In a method for lithographing a metal which can be passivated, wherein a protective layer is deposited on a first metal, and the first metal is etched by an acid solution which the protective layer resists, the improvement comprising:

depositing over the protective layer and the unprotected area of the first metal a layer of an auxiliary metal which is more oxidizable than the first so as to depassivate said first metal, and

etching said auxiliary metal with an acid, which the protective layer resists, immediately prior to the acid etching of the first metal so as to completely remove said auxiliary metal.

2. The method according to claim 1, wherein the first metal is platinum and the auxiliary metal is silver.

3. In a method for the selective metallizing of an electronic semiconductor device by a first metal which can be passivated, wherein the first metal is deposited non-selectively on the device, a protective layer is deposited selectively on the first metal, and the first metal is etched by an acid solution which the protective layer resists, the improvement comprising the steps of depositing on the protective layer and thefunprotected areas of the first metal, an auxiliary metal that is more oxidizable than the first metal after the selective depositing of the protective layer and before the etching of said first metal so as to depassivate said first metal, and acid etching said auxiliary metal with an acid which the protective layer resists immediately prior to the acid etching of the first metal so as to completely remove said auxiliary metal.

4. The method according to claim 3, wherein the auxiliary metal is silver.

5. In a method for producing non-rectifying electrical contacts on an electronic silicon component, comprising the steps of:

forming a window in a silicon oxide layer coating the silicon;

producing a silicon-platinum alloy in said window;

depositing titanium on an area comprising that window;

depositing platinum on said area;

selectively depositing a protective layer on a contact area inside said area and comprising said window;

etching said platinum deposit, said etching being prevented on said contact area by the protective layer;

removing the protective layer;

depositing gold on said contact area;

soldering a gold wire on the gold deposit; and

removing the titanium deposit outside said contact area subsequent to the etching of the platinum deposit;

the improvement comprising the step of:

depositing silver on the unprotected areas of the platinum deposit and the protective layer after the depositing of the protective layer and before etching of the platinum so as to depassivate said platinum, and

acid etching said silver with an acid which the protective layer resists immediately prior to said platinum etching step so as to completely remove said silver. 

1. In a method for lithographing a metal which can be passivated, wherein a protective layer is deposited on a first metal, and the first metal is etched by an acid solution which the protective layer resists, the improvement comprising: depositing over the protective layer and the unprotected area of the first metal a layer of an auxiliary metal which is more oxidizable than the first so as to depassivate said first metal, and etching said auxiliary metal with an acid, which the protective layer resists, immediately prior to the acid etching of the first metal so as to completely remove said auxiliary metal.
 2. The method according to claim 1, wherein the first metal is platinum and the auxiliary metal is silver.
 3. In a method for the selective metallizing of an electronic semiconductor device by a first metal which can be passivated, wherein the first metal is deposited non-selectively on the device, a protective layer is deposited selectively on the first metal, and the first metal is etched by an acid solution which the protective layer resists, the improvement comprising the steps of depositing on the protective layer and the unprotected areas of the first metal, an auxiliary metal that is more oxidizable than the first metal after the selective depositing of the protective layer and before the etching of said first metal so as to depassivate said first metal, and acid etching said auxiliary metal with an acid which the protective layer resists immediately prior to the acid etching of the first metal so as to completely remove said auxiliary metal.
 4. The method according to claim 3, wherein the auxiliary metal is silver. 